If 'pattern' is an empty string, the last successfully matched
regex is used. Delimiters other than '/' may be used for both this
operator and the following ones. The leading m can be omitted
if the delimiter is '/'.

qr/pattern/msixpo lets you store a regex in a variable,
or pass one around. Modifiers as for m//, and are stored
within the regex.

CHARACTER CLASSES

The following sequences work within or without a character class.
The first six are locale aware, all are Unicode aware. See perllocale
and perlunicode for details.

\d A digit

\D A nondigit

\w A word character

\W A non-word character

\s A whitespace character

\S A non-whitespace character

\h An horizontal white space

\H A non horizontal white space

\v A vertical white space

\V A non vertical white space

\R A generic newline (?>\v|\x0D\x0A)

\C Match a byte (with Unicode, '.' matches a character)

\pP Match P-named (Unicode) property

\p{...} Match Unicode property with long name

\PP Match non-P

\P{...} Match lack of Unicode property with long name

\X Match extended Unicode combining character sequence

POSIX character classes and their Unicode and Perl equivalents:

alnum IsAlnum Alphanumeric

alpha IsAlpha Alphabetic

ascii IsASCII Any ASCII char

blank IsSpace [ \t] Horizontal whitespace (GNU extension)

cntrl IsCntrl Control characters

digit IsDigit \d Digits

graph IsGraph Alphanumeric and punctuation

lower IsLower Lowercase chars (locale and Unicode aware)

print IsPrint Alphanumeric, punct, and space

punct IsPunct Punctuation

space IsSpace [\s\ck] Whitespace

IsSpacePerl \s Perl's whitespace definition

upper IsUpper Uppercase chars (locale and Unicode aware)

word IsWord \w Alphanumeric plus _ (Perl extension)

xdigit IsXDigit [0-9A-Fa-f] Hexadecimal digit

Within a character class:

POSIX traditional Unicode

[:digit:] \d \p{IsDigit}

[:^digit:] \D \P{IsDigit}

ANCHORS

All are zero-width assertions.

^ Match string start (or line, if /m is used)

$ Match string end (or line, if /m is used) or before newline

\b Match word boundary (between \w and \W)

\B Match except at word boundary (between \w and \w or \W and \W)

\A Match string start (regardless of /m)

\Z Match string end (before optional newline)

\z Match absolute string end

\G Match where previous m//g left off

\K Keep the stuff left of the \K, don't include it in $&

QUANTIFIERS

Quantifiers are greedy by default -- match the longest leftmost.

Maximal Minimal Possessive Allowed range

------- ------- ---------- -------------

{n,m} {n,m}? {n,m}+ Must occur at least n times

but no more than m times

{n,} {n,}? {n,}+ Must occur at least n times

{n} {n}? {n}+ Must occur exactly n times

* *? *+ 0 or more times (same as {0,})

+ +? ++ 1 or more times (same as {1,})

? ?? ?+ 0 or 1 time (same as {0,1})

The possessive forms (new in Perl 5.10) prevent backtracking: what gets
matched by a pattern with a possessive quantifier will not be backtracked
into, even if that causes the whole match to fail.

There is no quantifier {,n} -- that gets understood as a literal string.

EXTENDED CONSTRUCTS

(?#text) A comment

(?:...) Groups subexpressions without capturing (cluster)

(?pimsx-imsx:...) Enable/disable option (as per m// modifiers)

(?=...) Zero-width positive lookahead assertion

(?!...) Zero-width negative lookahead assertion

(?<=...) Zero-width positive lookbehind assertion

(?<!...) Zero-width negative lookbehind assertion

(?>...) Grab what we can, prohibit backtracking

(?|...) Branch reset

(?<name>...) Named capture

(?'name'...) Named capture

(?P<name>...) Named capture (python syntax)

(?{ code }) Embedded code, return value becomes $^R

(??{ code }) Dynamic regex, return value used as regex

(?N) Recurse into subpattern number N

(?-N), (?+N) Recurse into Nth previous/next subpattern

(?R), (?0) Recurse at the beginning of the whole pattern

(?&name) Recurse into a named subpattern

(?P>name) Recurse into a named subpattern (python syntax)

(?(cond)yes|no)

(?(cond)yes) Conditional expression, where "cond" can be:

(N) subpattern N has matched something

(<name>) named subpattern has matched something

('name') named subpattern has matched something

(?{code}) code condition

(R) true if recursing

(RN) true if recursing into Nth subpattern

(R&name) true if recursing into named subpattern

(DEFINE) always false, no no-pattern allowed

VARIABLES

$_ Default variable for operators to use

$` Everything prior to matched string

$& Entire matched string

$' Everything after to matched string

${^PREMATCH} Everything prior to matched string

${^MATCH} Entire matched string

${^POSTMATCH} Everything after to matched string

The use of $`
, $&
or $'
will slow down all regex use
within your program. Consult perlvar for @-
to see equivalent expressions that won't cause slow down.
See also Devel::SawAmpersand. Starting with Perl 5.10, you
can also use the equivalent variables ${^PREMATCH}
, ${^MATCH}
and ${^POSTMATCH}
, but for them to be defined, you have to
specify the /p (preserve) modifier on your regular expression.

$1, $2 ... hold the Xth captured expr

$+ Last parenthesized pattern match

$^N Holds the most recently closed capture

$^R Holds the result of the last (?{...}) expr

@- Offsets of starts of groups. $-[0] holds start of whole match

@+ Offsets of ends of groups. $+[0] holds end of whole match

%+ Named capture buffers

%- Named capture buffers, as array refs

Captured groups are numbered according to their opening paren.

FUNCTIONS

lc Lowercase a string

lcfirst Lowercase first char of a string

uc Uppercase a string

ucfirst Titlecase first char of a string

pos Return or set current match position

quotemeta Quote metacharacters

reset Reset ?pattern? status

study Analyze string for optimizing matching

split Use a regex to split a string into parts

The first four of these are like the escape sequences \L
, \l
,
\U
, and \u
. For Titlecase, see Titlecase.

TERMINOLOGY

Titlecase

Unicode concept which most often is equal to uppercase, but for
certain characters like the German "sharp s" there is a difference.